Silicon MOSFET scaling has become a major challenge in the semiconductor industry. Traditional techniques start to fail in reducing certain undesirable physical effects as device dimensions shrink down to the nanometer regime. For example, anti-punchthrough (APT) or halo implantation is used to reduce the short-channel effects (SCE). However, the abrupt doping profiles are difficult to achieve due to temperature enhanced diffusion, and these highly doped channels or pocket implant regions increase junction capacitance and band-to-band tunneling. It has been shown by S. Thompson, et al., in “MOS scaling: transistor challenges for the 21st century,” Intel Technology Journal, Q3, 1998, that channel engineering can only decrease the circuit gate delay by ˜10% for a given technology, and it cannot provide channel length scaling for generation after generation that gate oxide and source/drain (S/D) junction depth scaling has provided.
With bandgap engineering, an important degree of freedom can be provided in the device design. The growth of high-quality tensile strained Si/SiGe and compressively strained SiGe/Si heterostructures by molecular beam epitaxy (MBE), various types of chemical vapor deposition (CVD), and/or ion implantation allows incorporation of bandgap engineering concepts into a mature silicon technology.
Bandgap engineering has been utilized to realize various types of heterojunction field effect transistors (HFETs). The most widely studied is the modulation doped field effect transistors (MODFET), in which a quantum well is used to confine the carriers in a lightly doped semiconductor (See K. Ismail, “Si/SiGe High-Speed Field-Effect Transistors”, IEDM, Technology Digest, p. 509-512, 1995). Higher carrier mobility can be achieved due to reduced impurity scattering, reduced surface roughness scattering in the buried channel, and strained-induced mobility enhancement, if any, depending on the hetero material system employed. Derived from the same concept, various types of heterostructure CMOS devices have also been proposed and studied (See M. A. Armstong, et al., “Design of Si/SiGe Heterojunction Complementary Metal-Oxide Semiconductor Transistors”, IEDM Technology Digest, p. 761-764, 1995; S. Imai et al., “Si—SiGe Semiconductor Device and Method of Fabricating the Same”, U.S. Pat. No. 5,847,419; and M. Kubo, et al., “Method of Forming HCMOS Devices with a Silicon-Germanium-Carbon compound Semiconductor Layer”, U.S. Pat. No. 6,190,975, Feb. 20, 2001.) The advantage of these devices is the higher carrier mobility and hence high drive current and high speed. However, two prominent problems still remain in these planar devices: device scaling and control of short-channel effects.
In planar FET devices, the channel length is limited by lithography. This problem can be solved if the devices are fabricated in a vertical fashion, in which case the channel length is only determined by epitaxial techniques. Similarly, the diffusion problem of boron and phosphorus in the source/drain can be reduced by introducing thin SiGeC layers in the source/drain to achieve ultra scalable vertical transistors, as shown by Y. Ming, et al., in “25-nm p-Channel vertical MOSFET's with SiGeC source-drains”,IEEE, Electron Device Letters, vol. 20, no. 6, 1999, and by H. Rücker et al., in “Dopant diffusion in C-doped Si and SiGe: physical model and experimental verification,” IEDM, Technical Digest, p. 345-8, 1999.
As for short-channel effects, other than ultra-steep retrograded channel profiles and ultra-shallow source/drain junctions, silicon-on-insulator (SOI) has been used to control short-channel effects. However, SOI does not remove short-channel effects completely, and an inherent problem with SOI is the floating body effect. Another way to reduce the short-channel effect is to have a built-in energy barrier at the source/body junction, and in particular a barrier where the barrier height does not depend on the applied bias. The band offset provided by a heterojunction is very suitable in this case. A heterojunction MOSFET (HJMOSFET) has been proposed and studied by S. Hareland, et al., in “New structural approach for reducing punchthrough current in deep submicrometer MOSFETs and extending MOSFET scaling,” IEEE Electronics Letters, vol. 29, no. 21, pp. 1894-1896, October, 1993, and by X. D. Chen, et al., in “Vertical P-MOSFETS with heterojunction between source/drain and channel,” Device Research Conference, Denver, June, 2000.
A p-channel/n-channel, complementary vertical MISFET device and a specific application of such devices in dynamic random access memory (DRAM) are described in U.S. Pat. No. 5,920,088, No. 6,207,977, No. 5,963,800, and No. 5,914,504, respectively. A heterojunction is utilized at the source/channel junction in the vertical devices. Even though very short channels may be achieved and short-channel effects may be reduced, there is still a big drawback with such device structures. At the off state (i.e., zero bias at the gate and high bias at the drain), the hetero-barrier is useful in reducing the drain-induced barrier lowering (DIBL), bulk punchthrough and therefore, off-state leakage current. However, at the on state (i.e., high bias at the gate and drain), the built-in hetero-barrier becomes harmful to the drive current. This is because the hetero-barrier at the source/channel junction severely blocks the thermal emission of the carriers from the source into the channel. The only way for carrier injection is the quantum mechanical tunneling across the barrier, which becomes the bottleneck of the transport in the channel. The so-called ballistic transport after crossing the barrier in the channel mentioned in these references will not occur due to strong surface roughness scattering. Therefore, the drive current in such devices is significantly reduced. Additionally, since a part of the source (close to the channel) of such a device is undoped, the drive current will be further reduced by high series resistance in the source. A detailed study has been performed by Q. Ouyang, et al., in “Two-Dimensional Bandgap Engineering in Novel pMOSFETs,” SISPAD, Seattle September, 2000, and by X. D. Chen, et al., in “Vertical P-MOSFETS with heterojunction between source/drain and channel”, Device Research Conference, Denver, June, 2000.
Recently, a lateral high mobility, buried, p-channel heterojunction transistor (HMHJT) has been proposed by Q. Ouyang, et al., in U.S. Pat. No. 6,319,799B1. A detailed simulation study has been performed by Q. Ouyang, et al., in “A Novel Si/SiGe Heterojunction pMOSFET with Reduced Short-Channel Effects and Enhanced Drive Current,” IEEE Transactions on Electron Devices, 47 (10), 2000. The device has been realized using a vertical structure by Q. Ouyang, et al., in “Fabrication of a Novel Vertical pMOSFET with Enhanced Drive Current and Reduced Short-Channel Effects and Floating Body Effects”, VLSI Symposium, Kyoto, June 2001. In this case, compressively strained SiGe on Si is used to realize a high performance pMOSFET. However, in order to realize circuits using complementary MOSFETs, a high-performance, vertical nMOSFET is also needed. In the present invention, we propose a heterojunction nMOSFET, which has low leakage and high drive current. Six embodiments are illustrated and the methods thereof are also described.
U.S. Pat. No. 5,285,088 describes a “High Electron Mobility Transistor”. This device has a pair of semiconductor layers for source/drain electrodes consisting of a poly SiGe layer and apoly Si layer so as to form a partially projected “overhanging-shape” over the active area. In this case, the source/drain and the gate are self-aligned. However, it is a planar structure and still suffers from the short-channel effects.